1. Technical Field
The present disclosure relates to a position indicator of an electromagnetic induction type which is used in conjunction with a position detection device that detects a position indicated by the position indicator using electromagnetic inductive coupling, and to a position indicating method.
2. Description of the Related Art
A position indicator of this type which is widely used today includes a resonant circuit formed by a parallel circuit composed of a coil and a capacitor, and is configured to return electromagnetic waves transmitted from a position detection device to the position detection device through the resonant circuit. In the position indicator of this type, the characteristics of the resonant circuit are often controlled in accordance with transmission data representing, for example, a pen pressure in synchronization with electromagnetic waves transmitted intermittently from the position detection device to return the transmission data representing, for example, the pen pressure to the position detection device (see, for example, Japanese Patent Laid-Open No. 2005-10844).
FIG. 10 is a diagram illustrating an exemplary configuration of the position indicator of this type, and FIGS. 11A to 11D are timing diagrams illustrating waveforms of signals at various portions of the position indicator having the exemplary configuration illustrated in FIG. 10.
A position indicator 1 illustrated in FIG. 10 includes a resonant circuit 2 formed by a parallel circuit composed of a coil 2L and a capacitor 2C connected in parallel. In the resonant circuit 2 according to this example, a switch 2S is connected in parallel with the coil 2L and the capacitor 2C. In this example, one end of the coil 2L of the resonant circuit 2 is grounded, while induction signals b (see FIG. 11B) based on electromagnetic waves a (see FIG. 11A) transmitted intermittently from a position detection device are obtained at an opposite end of the coil 2L.
Signals transmitted from the position detection device in the form of the electromagnetic waves a are alternating-current signals having a frequency equal to a resonance frequency of the resonant circuit 2 of the position indicator 1, and the alternating-current signals are made up of a signal (hereinafter referred to as a burst signal) which continues for 500 microseconds, for example, and data transmission synchronizing signals each of which continues for a period, e.g., 50 microseconds, shorter than the duration of the burst signal, and the number of which corresponds to the number of bits of transmission data representing, for example, a so-called pen pressure, i.e., a pressure applied to a tip portion (a pen point) of the position indicator 1, so that the transmission data can be transmitted by the position indicator 1. The data transmission synchronizing signals are synchronizing signals used when data is exchanged between the position indicator 1 and the position detection device, and are used, in the position detection device, to detect the transmission data from the position indicator 1 through sampling. In this case, the position detection device repeats a cycle of transmission of the burst signal and the subsequent data transmission synchronizing signals, the number of which is equal to the number of bits of the data transmitted from the position indicator 1.
The induction signals b obtained at the aforementioned opposite end of the coil 2L of the resonant circuit 2 due to the electromagnetic waves a transmitted from the position detection device are supplied to a detector circuit 3. In the detector circuit 3, envelope detection outputs for the induction signals b are each compared with a predetermined threshold value to generate a timing signal c (see FIG. 11C) which is synchronized with the electromagnetic waves a from the position detection device. The timing signal c generated in the detector circuit 3 is supplied to a control circuit 4, which is formed by, for example, a microprocessor.
The induction signal b obtained at the aforementioned opposite end of the coil 2L of the resonant circuit 2 is also supplied to and rectified by a rectifier circuit 5, and a charge storage capacitor 6 such as, for example, an electric double-layer capacitor, is charged with the rectified signal. The charge storage capacitor 6 forms a power supply circuit that generates a power supply voltage for driving the control circuit 4, and the control circuit 4 operates using an output voltage of the charge storage capacitor 6 as a power supply voltage Vcc.
The position indicator 1 illustrated in FIG. 10 includes a variable resistor 7 the resistance value of which varies in accordance with the pen pressure, and the control circuit 4 detects a voltage according to the resistance value of the variable resistor 7 to detect the pen pressure. Then, the control circuit 4 converts the detected pen pressure into multiple bits of digital data, and supplies a control signal d (see FIG. 11D) according to each bit (“0” or “1”) of the digital data to the switch 2S to control the switch 2S to be turned on and off.
More specifically, in the example of FIG. 10, when a bit of the digital data, representing the pen pressure, is “1,” the switch 2S is turned on to short both ends of the coil 2L so that electromagnetic wave energy stored in the coil 2L of the resonant circuit 2 disappears to cause no electromagnetic wave to be returned from the position indicator 1 to the position detection device. Meanwhile, when a bit of the digital data, representing the pen pressure, is “0,” the switch 2S is kept in an OFF state to cause an electromagnetic wave to be returned from the position indicator 1 to the position detection device through the resonant circuit 2 composed of the coil 2L and the capacitor 2C. The digital data representing, for example, the pen pressure is thus subjected to amplitude shift keying (ASK) modulation or on-off keying (OOK) modulation, and is returned from the position indicator 1 to the position detection device.
In the position detection device, the electromagnetic waves subjected to the ASK modulation or the OOK modulation and returned from the position indicator 1 are sampled at a sampling timing based on the transmitted electromagnetic waves a, and when no electromagnetic wave is returned from the position indicator 1 at the sampling timing, the bit of the digital data is determined to be “1,” and when the electromagnetic wave returned from the position indicator 1 at the sampling timing has a signal level equal to or higher than a predetermined threshold value, the bit of the digital data is determined to be “0.” The digital data is thus demodulated.
As described above, the position indicator 1 illustrated in FIG. 10 generates the power supply voltage for driving the control circuit 4 from the electromagnetic waves transmitted from the position detection device, and also subjects the electromagnetic waves received from the position detection device to the ASK modulation or the OOK modulation using the digital data to be transmitted, and returns the modulated electromagnetic waves to the position detection device.
As described above, the position detection device repeatedly and cyclically transmits the alternating-current signals having a frequency equal to the resonance frequency of the resonant circuit 2 of the position indicator 1, that is, the burst signal which continues for a relatively long period, and the subsequent data transmission synchronizing signals each of which continues for a relatively short period and the number of which is equal to the number of bits of the transmission data to be transmitted from the position indicator 1. On the part of the position indicator 1, a charge storage capacitor 6 is charged primarily with the burst signal of the electromagnetic waves transmitted from the position detection device, and the data transmission synchronizing signals are subjected to the ASK modulation or the OOK modulation using the transmission data representing, for example, the pen pressure to transmit the transmission data to the position detection device.
Here, from the viewpoint of emphasizing the charging of the charge storage capacitor 6 in the position indicator 1, it is important to configure the resonant circuit 2 so as to minimize a loss of energy of the electromagnetic waves transmitted from the position detection device. Meanwhile, in view of transmitting the transmission data from the position indicator 1 to the position detection device, it is important to configure the position indicator 1 so as to be capable of increasing the speed of signal transmission in order to satisfy a recent demand for an increased number of bits of transmission data.
However, reducing the energy loss and increasing the speed of signal transmission are generally incompatible with each other, and to configure the resonant circuit of the position indicator so as to satisfy both the demands has been difficult so far.
In more detail, when the switch 2S is in the OFF state in FIG. 10, the resonant circuit 2 can be considered to have an infinitely large load resistance, and the resonant circuit 2 has a load resistor having a high resistance connected thereto. When the resonant circuit 2 thus has a large load resistance, an energy loss is small, but the resonant circuit 2 has a large resonance sharpness (Q), resulting in a reduction in the speed of signal transmission. Meanwhile, when the resonant circuit 2 has a small load resistance, the resonant circuit 2 has a small resonance sharpness (Q), which results in an increase in the speed of signal transmission, but a large energy loss occurs.